Internal combustion engines typically use either a mechanical, electrical, or hydro-mechanical valve actuation system to actuate the engine valves. These systems may include a combination of camshafts, rocker arms and push rods that are driven by the engine's crankshaft rotation. When a camshaft is used to actuate the engine valves, the timing of the valve actuation may be fixed by the size and location of the lobes on the camshaft.
For each 360 degree rotation of the camshaft, the engine completes a full cycle made up of four strokes (i.e., expansion, exhaust, intake, and compression). Both the intake and exhaust valves may be closed, and remain closed, during most of the expansion stroke wherein the piston is traveling away from the cylinder head (i.e., the volume between the cylinder head and the piston head is increasing). During positive power operation, fuel is burned during the expansion stroke and positive power is delivered by the engine. The expansion stroke ends at the bottom dead center point, at which time the piston reverses direction and the exhaust valve may be opened for a main exhaust event. A lobe on the camshaft may be synchronized to open the exhaust valve for the main exhaust event as the piston travels upward and forces combustion gases out of the cylinder. Near the end of the exhaust stroke, another lobe on the camshaft may open the intake valve for the main intake event at which time the piston travels away from the cylinder head. The intake valve closes and the intake stroke ends when the piston is near bottom dead center. Both the intake and exhaust valves are closed as the piston again travels upward for the compression stroke.
The above-referenced main intake and main exhaust valve events are required for positive power operation of an internal combustion engine. Additional auxiliary valve events, while not required, may be desirable. For example, it may be desirable to actuate the intake and/or exhaust valves during positive power or other engine operation modes for compression-release engine braking, bleeder engine braking, exhaust gas recirculation (EGR), brake gas recirculation (BGR), or other auxiliary intake and/or exhaust valve events. FIG. 6 illustrates examples of a main exhaust event 700, and auxiliary valve events, such as a compression-release engine braking event 710, bleeder engine braking event 720, exhaust gas recirculation event 740, and brake gas recirculation event 730, which may be carried out by an engine valve using various embodiments of the present invention to actuate engine valves for main and auxiliary valve events.
With respect to auxiliary valve events, flow control of exhaust gas through an internal combustion engine has been used in order to provide vehicle engine braking. Generally, engine braking systems may control the flow of exhaust gas to incorporate the principles of compression-release type braking, exhaust gas recirculation, exhaust pressure regulation, and/or bleeder type braking.
During compression-release type engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, a power producing internal combustion engine into a power absorbing air compressor. As a piston travels upward during its compression stroke, the gases that are trapped in the cylinder may be compressed. The compressed gases may oppose the upward motion of the piston. As the piston approaches the top dead center (TDC) position, at least one exhaust valve may be opened to release the compressed gases in the cylinder to the exhaust manifold, preventing the energy stored in the compressed gases from being returned to the engine on the subsequent expansion down-stroke. In doing so, the engine may develop retarding power to help slow the vehicle down. An example of a prior art compression release engine brake is provided by the disclosure of the Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is hereby incorporated by reference.
During bleeder type engine braking, in addition to, and/or in place of, the main exhaust valve event, which occurs during the exhaust stroke of the piston, the exhaust valve(s) may be held slightly open during the remaining three engine cycles (full-cycle bleeder brake) or during a portion of the remaining three engine cycles (partial-cycle bleeder brake). The bleeding of cylinder gases in and out of the cylinder may act to retard the engine. Usually, the initial opening of the braking valve(s) in a bleeder braking operation is in advance of the compression TDC (i.e., early valve actuation) and then lift is held constant for a period of time. As such, a bleeder type engine brake may require lower force to actuate the valve(s) due to early valve actuation, and generate less noise due to continuous bleeding instead of the rapid blow-down of a compression-release type brake.
Exhaust gas recirculation (EGR) systems may allow a portion of the exhaust gases to flow back into the engine cylinder during positive power operation. EGR may be used to reduce the amount of NOx created by the engine during positive power operations. An EGR system can also be used to control the pressure and temperature in the exhaust manifold and engine cylinder during engine braking cycles. Generally, there are two types of EGR systems, internal and external. External EGR systems recirculate exhaust gases back into the engine cylinder through an intake valve(s). Internal EGR systems recirculate exhaust gases back into the engine cylinder through an exhaust valve(s) and/or an intake valve(s). Embodiments of the present invention primarily concern internal EGR systems.
Brake gas recirculation (BGR) systems may allow a portion of the exhaust gases to flow back into the engine cylinder during engine braking operation. Recirculation of exhaust gases back into the engine cylinder during the intake stroke, for example, may increase the mass of gases in the cylinder that are available for compression-release braking. As a result, BGR may increase the braking effect realized from the braking event.
During operation of an engine, beginning from a cold start, certain engine components heat up and may experience thermal expansion. Additionally, over the life of an engine, engine components may wear, and thus change size and shape. Engine poppet valves and the systems used to actuate them are exposed to significant temperature changes and potential wear, and accordingly, these systems must allow for thermal growth and other phenomena that may affect actuation of the engine valves. Historically, thermal growth and the like have been accommodated by providing a lash space between the engine valve (or a valve bridge that spans two or more engine valves) and the valve actuator, such as a rocker arm, cam, push tube, and the like. This lash space has been set manually, or in some cases, automatically, using hydraulic lash adjusters between the engine valve and the valve actuator.
Hydraulic lash adjustors, however, have not been used to automatically adjust lash space between an engine valve and a valve actuation system designed to provide both positive power and auxiliary engine valve events, such as engine braking events. Accordingly, lash has been set manually in engines equipped with compression-release or bleeder type engine brakes. Manually setting lash may be a cumbersome and expensive process required both at the factory during manufacturing and in service. A system for hydraulically adjusting lash in engines equipped with an engine brake may reduce or even eliminate the need for automatic lash setting machines at the factory, cutting production time and assembly cost. Further, such systems may reduce maintenance needs and thereby provide even more savings.
An advantage of some, but not necessarily all, embodiments of the present invention may result from providing a hydraulic lash adjustor of the type described herein in systems that provide both positive power and auxiliary valve events. For example, it is not uncommon for engine valve float to occur as the result of an over speed condition or high exhaust backpressure in the engine. In such situations, a conventional hydraulic lash adjuster may “jack” by progressively locking excess hydraulic fluid in the lash adjustment circuit such that the engine valve at issue does not close properly even when the cam actuating it is at base circle. Unlike these conventional hydraulic lash adjusters, embodiments of the invention may be largely impervious to jacking due to the operation of the motion absorbing piston.